Why does fluid (water) stick to the top of an obstacle?
The no-slip for the obstacle is incorrect. No-slip as in not slippery so it is sticky.

From the wiki page on obstacles -


Noslip causes the fluid to stick to the obstacle (zero velocity).
Free(-slip) allows movement along the obstacle (only zero normal velocity).
Part(-slip) mixes both types, with 0 being mostly noslip, and 1 being identical to freeslip.